1. Field of the Invention
The present invention relates to a rolling mill with an upper housing which is vertically movable through a column in relation to a lower housing, and a method of operating the rolling mill.
2. Description of the Prior Art
As a conventional reduction device of a rolling mill, a SCREW-type reduction device (a conventional example 1) shown in FIG. 8 and a hydraulic reduction device (a conventional example 2) disclosed in Japanese Patent Laid-open No. Hei 3-169406 (European Patent No. 0 429 812 A2).
In FIG. 8, a lower housing 31 holding lower rolls 33 is securely set on a base 32, and an upper housing 36 holding upper rolls 37 is vertically movably mounted on four columns 34 which are installed upright through nuts 35 at four corners of the lower housing 31. Each of the columns 34 is decreased in diameter at the top end, which is provided with a threaded portion 38. The threaded portion 38 is installed in a threaded hole 40 of a worm wheel 39 which is rotatably but vertically unmovably fitted in the upper housing 36. A worm 41 in mesh with each worm wheel 39 is synchronously rotated by a driving gear and a transmission gear (not illustrated), thereby moving the upper housing 36 up and down to preset a roll gap and to widen the roll gap at the time of roll replacement.
The conventional example 1 stated above, however, has the following drawback that the upper rolls can not be raised rapidly when the rapid widening of the roll gap is required at the time of strip or roll replacement because the elevating speed of the upper housing is 60 mm/min., with the result that a fire will arise in case of a plate fracture or a fractured material will stay in a roll tool section, and also that the housing will take considerable time when lowering to preset the roll gap, presenting a problem in operation efficiency.
On the other hand, in the conventional example 2, the upper housing elevating means is a hydraulic elevating means; therefore, the rolling mill of the example 2, capable of, for example, rapid widening of the roll gap, is much more advantageous than the conventional example 1.
However, the conventional example 2 has the problem that since the reduction force of the rolls is hydraulically adjusted, plate thickness control accuracy will become deteriorated due to fluid compressibility.